Breast cancer screening clinics have been established to help in early detection of breast cancer, a major cause of death in women where asymptomatic women are encouraged to come to the clinic for annual examinations. When a suspected lesion is found, the woman is directed to see her physician for treatment.
No single method of examination has been found to give 100% detection probability. Instead, a combination of tests is performed, each of which probes a different aspect of possible breast cancer. The first is a check of family history for possible genetic susceptibility to breast cancer. The second is a physical examination (palpation) for detecting growths large enough to be felt. The third is mammography (soft x-ray imaging) for identifying tumors, whose increased density over normal tissue results in increased x-ray absorption. The fourth is thermography, which uses an infrared camera and display system to detect hot spots, indicative of abnormally high blood flow, an area characteristic of a tumor.
Even the above combination of tests, while more effective than any single test, is not successful in detecting every tumor. However, it has been discovered that ultrasonic imaging is ideally suited for examination of soft tissue structure such as the breast. Ultrasound is sensitive to composition, density, elasticity, shape and orientation. Since a tumor differs in these respects with the host medium, it should be detected with ultrasonic imaging. Because ultrasonic wavelengths of less than 1 mm can effectively penetrate the breast, tumors as small as several mm in diameter can be detected.
Pulse-echo ultrasonic imaging techniques have been applied in medical diagnoses for over twenty years. This type of imaging is especially attractive for medical applications for the following reasons:
(A) The low acoustic intensities used in diagnostic ultrasound are typically less than 10 mW/cm.sup.2, which are several orders of magnitude below the threshold intensities for damage to sensitive tissues and organs.
(B) This type of imaging has particular facility in the display of soft tissue structures. Because the absorption of ultrasound by muscle and fat is typically in the range of 0.5- 2.0 db/MH/cm, at 2.25 MH, ultrasound passing through fatty tissue is attenuated by approximately 10 db at 10 cm.
A tumor embedded in normal tissue will reflect a different pulse and in general, this echo pulse will differ in all respects from its surroundings.
The simplest form of imagining is the A-scan, or time-amplitude ultrasonography. This type of scan system contains an ultrasonic transducer and a scanning device which includes a pulse generator, a receiver-processor, and a monitor oscilloscope. To record an A-scan, the transducer is first placed on the area of interest or coupled to it by a suitable coupling medium such as water. A high voltage pulse is applied to the transducer, which emits a burst of ultrasound that enters the target. The transducer is well damped for good axial resolution, and in general contains a spherical lens to focus the ultrasonic output to a thin beam. A portion of the incident ultrasonic beam is backscattered whenever the beam encounters an interface between two media having different acoustic impedences (due to different densities or elasticities) or passes through an inhomogeneous (heterogeneous) medium.
The backscattered sound, i.e. echoes, is reflected back to the transducer and is processed by the receiver and displayed on the monitor as a train of pulses. The amplitude of a pulse is proportional to the amplitude of the reflected ultrasound, while the position and time of the pulse is proportional to the distance of the corresponding echo producing interface from the transducer (provided that the propagation velocity is constant). Therefore, this type of scan has the capacity to indicate that the tumor is present in normal tissue, however, its exact position, shape and form cannot be determined.
A more comprehensive form of ultrasonic imaging is a B-scan, or ultrasonic tomogram. This method involves scanning a transducer across the target, by translation of the transducer alone or by both translating and rotating the transducer. As was true in the A-scan, echoes picked up by the B-scan transducer are processed by the receiver. However, the amplified echoes are fed to a monitor unit which generates a series of dots corresponding to the echoes received. The horizontal and vertical imputs of the monitor unit are controlled by position and angle sensing potentiometers linked to the movement of the transducer. Therefore, at any given moment, the position and orientation of the transducer are duplicated on the display. The dots then represent interfaces encountered by the ultrasonic beam at that particular orientation and a continuous scan generates a complete cross section which can be photographed in a time exposure or viewed on a storage screen.
However, since the B-scan method of ultrasonography only produces cross-sectional images, the location of a possible breast tumor at an arbitrary site would require a time consuming series of parallel and closely spaced scanning operations, typically on the order of 100 scans. Clearly then another method of ultrasonic imaging is needed which would reduce the required number of views.
A B-scan method has been developed which reduces the number of scans to approximately 20. This method is called ultrasonic stereography and involves halographic processing of the B-scans before viewing the final image. The reconstructed image to be truly three-dimensional can be viewed naturally with illumination from a Tensor-type light source, without the need for a laser or a stereo viewer. The physician can look at the breast from different viewpoints (using the virtual image reconstructed by the stereohologram) or can selectively examine any plane through the breast (by placing a ground glass screen in the real image reconstructed by the stereohologram). However, this method still requires a significant number of scans, does not give instantaneous results, and requires a step of holographic processing.